US20080152363A1 - Polarization tracking and signal equalization for optical receivers configured for on-off keying or pulse amplitude modulation signaling - Google Patents
Polarization tracking and signal equalization for optical receivers configured for on-off keying or pulse amplitude modulation signaling Download PDFInfo
- Publication number
- US20080152363A1 US20080152363A1 US11/959,960 US95996007A US2008152363A1 US 20080152363 A1 US20080152363 A1 US 20080152363A1 US 95996007 A US95996007 A US 95996007A US 2008152363 A1 US2008152363 A1 US 2008152363A1
- Authority
- US
- United States
- Prior art keywords
- digital
- vectors
- optical
- sequence
- optical carrier
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 230000003287 optical effect Effects 0.000 title claims abstract description 162
- 230000010287 polarization Effects 0.000 title claims abstract description 70
- 230000011664 signaling Effects 0.000 title claims description 16
- 239000013598 vector Substances 0.000 claims abstract description 73
- 238000005070 sampling Methods 0.000 claims abstract description 19
- 238000000034 method Methods 0.000 claims description 15
- 230000005540 biological transmission Effects 0.000 claims description 14
- 239000000203 mixture Substances 0.000 claims description 5
- 230000001902 propagating effect Effects 0.000 claims 1
- 238000012545 processing Methods 0.000 description 30
- 238000010586 diagram Methods 0.000 description 13
- 230000006735 deficit Effects 0.000 description 11
- 239000006185 dispersion Substances 0.000 description 10
- 239000000835 fiber Substances 0.000 description 6
- 230000001419 dependent effect Effects 0.000 description 5
- 230000003595 spectral effect Effects 0.000 description 4
- 238000012549 training Methods 0.000 description 4
- 230000008859 change Effects 0.000 description 3
- 230000009268 pathologic speech processing Effects 0.000 description 3
- 230000008569 process Effects 0.000 description 3
- 208000032207 progressive 1 supranuclear palsy Diseases 0.000 description 3
- 230000003044 adaptive effect Effects 0.000 description 2
- 239000000969 carrier Substances 0.000 description 2
- 230000001427 coherent effect Effects 0.000 description 2
- 230000005684 electric field Effects 0.000 description 2
- 230000009021 linear effect Effects 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 230000007246 mechanism Effects 0.000 description 2
- 239000013307 optical fiber Substances 0.000 description 2
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 230000002238 attenuated effect Effects 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 238000012937 correction Methods 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 238000002372 labelling Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000009022 nonlinear effect Effects 0.000 description 1
- 230000010363 phase shift Effects 0.000 description 1
- 238000003672 processing method Methods 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 238000007619 statistical method Methods 0.000 description 1
- 230000002123 temporal effect Effects 0.000 description 1
- 238000000411 transmission spectrum Methods 0.000 description 1
Images
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
- H04B10/695—Arrangements for optimizing the decision element in the receiver, e.g. by using automatic threshold control
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
- H04B10/697—Arrangements for reducing noise and distortion
- H04B10/6971—Arrangements for reducing noise and distortion using equalisation
Definitions
- the present invention relates to optical communication equipment and, more specifically, to signal processing in optical receivers of on-off keying (OOK) and/or pulse amplitude modulation (PAM) signals.
- OOK on-off keying
- PAM pulse amplitude modulation
- a polarization-diverse intradyne receiver operates on an optical signal having two independently modulated polarization components.
- the receiver may employ sophisticated signal processing to recover the two corresponding data streams.
- a local laser also often referred to as a reference optical carrier or local oscillator
- it is very difficult to align the polarization of the local laser with that of the received optical signal because the polarization of the latter is affected by the optical transmission link and tends to change over time.
- an optical receiver adapted to recover on-off keying (OOK) or pulse amplitude modulation (PAM) data carried by a modulated optical carrier comprises an optical detector adapted to produce a sequence of vector pairs having first and second digital vectors indicative of complex values of first and second polarization components, respectively, of the modulated optical carrier at a corresponding sampling time.
- the optical receiver further comprises a digital processor that is connected to receive the sequence and is adapted to perform a rotation on each pair in a manner that tends to compensate for polarization rotation produced by transmitting the modulated optical carrier from an optical transmitter thereof to the optical receiver.
- the digital processor is further adapted to estimate values of the OOK or PAM data encoded onto each of the first and second polarization components based on the vectors produced by the rotation in a manner responsive to values of energy errors in the estimated values.
- an apparatus of the invention comprises an optical receiver adapted to recover OOK or PAM data carried by a modulated optical carrier.
- the optical receiver comprises an optical detector adapted to produce a sequence of first digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time.
- the optical receiver further comprises a digital processor being connected to receive the sequence of said first digital vectors and being adapted to estimate values of said OOK or PAM data based on the received sequence of said first digital vectors in a manner responsive to values of energy errors in said estimated values.
- an apparatus of the invention comprises an optical receiver adapted to recover data carried by a modulated optical carrier.
- the optical receiver comprises an optical detector adapted to produce a sequence of vector pairs having first and second digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time, and each second digital vector being indicative of a complex value of a different second polarization component of the modulated optical carrier at the sampling time.
- the optical receiver further comprises a digital processor being connected to receive the sequence of said vector pairs and being adapted to perform a rotation on a pair from said sequence in a manner that tends to compensate for polarization rotation produced during transmission of the modulated optical carrier from an optical transmitter thereof to the optical receiver, the digital processor being configured to recover said data from vectors resulting from the performed rotation.
- a method of recovering OOK or PAM data carried by a modulated optical carrier comprises the step of producing a sequence of first digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time.
- the method further comprises the step of estimating values of said OOK or PAM data based on the sequence of said first digital vectors in a manner responsive to values of energy errors in said estimated values.
- a method of recovering data carried by a modulated optical carrier comprises the step of producing a sequence of vector pairs having first and second digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time, and each second digital vector being indicative of a complex value of a different second polarization component of the modulated optical carrier at the sampling time.
- the method further comprises the step of performing a rotation on a vector pair from said sequence in a manner that tends to compensate for polarization rotation produced during transmission of the modulated optical carrier from an optical transmitter thereof to an optical receiver, which is adapted to recover said data from vectors resulting from the performed rotation.
- FIG. 1 shows a representative optical transmission link
- FIG. 2 shows a block diagram of an optical receiver that can be coupled to the optical transmission link of FIG. 1 according to one embodiment of the invention
- FIG. 3 shows a block diagram of an optical-to-electrical (O/E) converter that can be used in the optical receiver of FIG. 2 according to one embodiment of the invention
- FIG. 4 shows a block diagram of a digital signal processor (DSP) that can be used in the optical receiver of FIG. 2 according to one embodiment of the invention
- FIG. 5 shows a block diagram of a polarization-tracking block that can be used in the DSP of FIG. 4 according to one embodiment of the invention
- FIG. 6 shows a block diagram of a signal-equalization block that can be used in the DSP of FIG. 4 according to one embodiment of the invention.
- FIGS. 7A-B graphically compare the performance of the receiver shown in FIG. 2 when it is configured to use (i) a prior-art algorithm and (ii) a signal-equalization algorithm according to one embodiment of the invention.
- FIG. 1 shows a representative optical transmission link 100 that couples an optical transmitter (Tx) and an optical receiver (Rx).
- the transmitter applies to link 100 an optical signal (modulated optical carrier) 102 having two independently modulated polarization components, illustratively a vertically polarized component 102 v and a horizontally polarized component 102 h.
- the transmitter generates both components of signal 102 using either on-off keying (OOK) or pulse amplitude modulation (PAM) signaling.
- OOK on-off keying
- PAM pulse amplitude modulation
- optical signal 152 having a vertically polarized component 152 v and a horizontally polarized component 152 h.
- Optical signal 152 is applied to the receiver, where appropriate signal processing is performed to recover the information encoded onto signal 102 at the transmitter.
- link 100 typically has an optical fiber. Different sections of the fiber typically have different principal states of polarization (PSPs) that are not necessarily aligned with the polarization components of signal 102 . This misalignment causes each of the polarization components to generally have a non-zero projection on each of the PSPs, which mixes the polarization components and alters the polarization state of the signal. In addition, the PSPs tend to change over time, e.g., due to varying physical conditions in link 100 . This temporal variation causes the relationship between signal components 102 v - h and 152 v - h to vary over time as well.
- PSPs principal states of polarization
- chromatic dispersion Another impairment that might be imposed onto optical signal 102 in link 100 is chromatic dispersion (CD), which is represented in FIG. 1 by block 120 .
- CD chromatic dispersion
- Two parameters commonly used to characterize chromatic dispersion are the first-order group velocity dispersion (GVD), which is measured in ps/km/nm, and the second-order GVD, which is measured in ps/km/nm 2 . In the optical domain, both the first- and second-order GVD are linear effects.
- the GVD becomes a nonlinear effect because a typical photo-detector measures the square of the electric field, and not the electric field itself.
- Chromatic dispersion is usually static and can be effectively compensated, as known in the art, by utilizing a special dispersion-compensation module.
- a special dispersion-compensation module might be relatively expensive.
- the dispersion-compensation module might add unwanted latency to the link's performance, which usually unfavorably affects the quality of service (QoS). It is also possible that the dispersion-compensation module does not fully cancel the chromatic dispersion accrued in optical link 100 , and signal 152 still has in it some residual amount of chromatic dispersion.
- PMD polarization-mode dispersion
- Fiber birefringence is typically caused by deviations in the shape of the fiber core from a perfect cylinder and can be induced, e.g., by stress, bending, and/or temperature gradients.
- the PMD phenomenon is dynamic in nature and the PMD amount tends to change over time.
- PMD is usually characterized by differential group delay (DGD), a parameter measured in ps/km 0.5 and related to PMD-induced optical pulse broadening.
- DTD differential group delay
- PMD is frequency-dependent.
- First-order PMD is a frequency-independent component of (this frequency-dependent) PMD and is responsible for linear inter-symbol interference (ISI) in the electrical domain (e.g., at the output of the photo-detector).
- ISI linear inter-symbol interference
- Second-order (as well as higher-order) PMD is a frequency-dependent component of PMD and is responsible for optical-pulse broadening similar to that produced by the corresponding order of chromatic dispersion.
- optical noise is represented in FIG. 1 by adders 140 v - h.
- Link 100 usually employs one or more optical amplifiers. While boosting the signal intensity, an optical amplifier might also add incoherent amplified spontaneous-emission (ASE) noise (often referred to as optical noise).
- ASE amplified spontaneous-emission
- the optical noise can be modeled as complex additive white Gaussian noise (AWGN) in the optical field.
- AWGN additive white Gaussian noise
- Adder 140 v indicates the addition of AWGN n v (t) to a vertically polarized component 132 v of an intermediate optical signal 132 .
- adder 140 h indicates the addition of AWGN n h (t) to a horizontally polarized component 132 h of signal 132 .
- link 100 might contain add/drop multiplexers, optical routers, and/or wavelength-division multiplexing (WDM) filters. Each of these elements is characterized by a corresponding transmission spectrum that affects the spectral content of an optical signal passing therethrough.
- WDM wavelength-division multiplexing
- impairments might accrue in link 100 through either localized or distributed mechanisms, or through a combination of both types of mechanisms.
- the order, in which the impairment blocks are shown in FIG. 1 is for illustration only. Neither is it intended to imply that different impairments are added in any particular order, nor that they accrue in separate physical sections of link 100 .
- FIG. 2 shows a block diagram of an optical receiver 200 that can be coupled to optical transmission link 100 ( FIG. 1 ) according to one embodiment of the invention.
- Receiver 200 has an O/E converter 220 adapted to convert optical signal 152 received from link 100 into electrical signals 238 a - d.
- Receiver 200 further has a plurality of amplifiers 240 , each receiving a corresponding one of electrical signals 238 a - d and being coupled to a corresponding analog-to-digital converter (ADC) 250 .
- ADC 250 samples the output of the corresponding amplifier 240 at sampling frequency f s to produce a corresponding one of digital signals 252 a - d.
- frequency f s is the same as the signaling rate in signal 152 or an integer multiple of that rate.
- Digital signals 252 a - d are applied to a digital signal processor (DSP) 260 that processes them to recover the two data streams originally encoded onto optical signal 102 at the transmitter (see FIG. 1 ). The recovered data streams are then output from DSP 260 via an output signal 262 .
- DSP digital signal processor
- O/E converter 220 implements polarization-sensitive intradyne detection using a reference signal 212 generated by a local oscillator 210 .
- Polarization beam splitters (PBSs) 222 a - b decompose signals 152 and 212 , respectively, into two respective orthogonally polarized components, illustratively vertically polarized components 152 v and 212 v and horizontally polarized components 152 h and 212 h. These polarization components are then directed to an optical hybrid 226 .
- O/E converter 220 is an integrated planar waveguide circuit.
- each of polarization components 152 v, 212 v, 152 h, and 212 h is split into two (attenuated) copies, e.g., using a conventional 3-dB power splitter.
- a relative phase shift of 90 degrees ( ⁇ /2 radian) is then applied to one copy of component 212 v and one copy of component 212 h using phase shifters 228 a - b, respectively.
- the various copies are optically mixed as shown in FIG. 2 using four optical signal mixers 230 , and the outputs of the mixers (the optical mixtures) are detected by eight photo-detectors (e.g., photodiodes) 236 .
- Photo-detectors 236 are arranged in pairs, as shown in FIG.
- electrical signal 238 a is a measure of the real part of vertically polarized component 152 v in the complex plane defined by LO signal 212 .
- electrical signal 238 b is a measure of the imaginary part of vertically polarized component 152 v in that complex plane;
- electrical signal 238 c is a measure of the real part of horizontally polarized component 152 h in that complex plane;
- electrical signal 238 d is a measure of the imaginary part of horizontally polarized component 152 h in that complex plane.
- DSP 260 substantially de-convolutes digital signals 252 a - d to recover the two data streams originally encoded onto signals 102 v - h.
- optical receiver 200 is capable of operating without employing conventional frequency and/or phase estimation for LO 210 because OOK and PAM signaling relies on photon energy (rather than phase) to carry bit information.
- FIG. 3 shows a block diagram of an O/E converter 320 that can be instead of O/E converter 220 of optical receiver 200 according to another embodiment of the invention.
- O/E converter 320 is generally analogous to O/E converter 220 and may be constructed using the same basic elements. However, O/E converter 320 is designed to perform optical signal mixing before polarization splitting, whereas O/E converter 220 performs polarization splitting before optical signal mixing.
- an optical hybrid 326 used in O/E converter 320 employs one phase shifter 228 , compared to two such phase shifters employed in optical hybrid 226 .
- Optical hybrid 326 also employs two optical signal mixers 230 , compared to four such mixers employed in optical hybrid 226 .
- O/E converter 320 employs four PBSs 222 , compared to two such PBSs employed in O/E converter 220 .
- optical hybrids 226 and 326 may be a bulk optical hybrid or a planar-waveguide optical hybrid. Suitable bulk optical hybrids are commercially available from Optoplex Corporation, of 3374-3390 Gateway Boulevard, Fremont, Calif. 94538.
- FIG. 4 shows a block diagram of a DSP 460 that can be used as DSP 260 according to one embodiment of the invention.
- DSP 460 has two serially connected processing blocks 470 and 480 .
- Processing block 470 is a polarization tracking (PolTrack) block designed to perform, in the electrical domain, adaptive polarization correction that substantially cancels the effect of polarization distortion in optical transmission link 100 (see block 110 in FIG. 1 ).
- Processing block 480 implements a signal equalization algorithm (SEA) that enables DSP 460 to recover the data carried by signal 152 .
- SEA signal equalization algorithm
- Processing block 470 operates on complex numbers.
- Input signal r v received by processing block 470 carries a stream of complex numbers, each composed of a real part and an imaginary part supplied by digital signals 252 a - b, respectively.
- input signal r h carries a stream of complex numbers, each composed of a real part and an imaginary part supplied by digital signals 252 c - d, respectively.
- Each of output signals z v and z h generated by processing block 470 similarly carries complex numbers. In the event that the orientation of principal polarization components of signal 152 of FIG.
- processing block 470 can be bypassed and input signals r v and r h can be applied directly to processing block 480 .
- the orientations of the polarization states of PBSs 222 and of principal polarization components of signal 152 can be aligned, for example, if the polarization distortion in optical transmission link 100 is relatively small or if an appropriately configured optical polarization rotator processes signal 152 before it is applied to receiver 200 .
- FIG. 5 shows a block diagram of a processing block 570 that can be used as processing block 470 of FIG. 4 according to one embodiment of the invention.
- the polarization distortion (see block 110 in FIG. 1 ) in the link consists substantially of polarization rotation and can be described using a Jones matrix.
- Processing block 570 has a vector rotator 572 that rotates signals r v and r h (which are vectors in the complex plane) by angle ⁇ , the value of which is chosen so as to substantially cancel the polarization rotation imposed in link 100 .
- Eq. (1) shows the corresponding Jones matrix used in vector rotator 572 and how it is applied to signals r v and r h to obtain signals z v and z h :
- k is a running index that denotes signaling intervals or time slots.
- Processing block 570 further has an error-tracking module (ETM) 574 configured to provide the ⁇ (k) values to vector rotator 572 .
- ETM 574 calculates signal errors (e(k)) for both vertical and horizontal polarizations using Eqs. (2a)-(2b):
- M For M-level PAM signaling, d v,h (k) or ⁇ circumflex over (d) ⁇ v,h (k) ⁇ 0,1, . . . , M ⁇ 1 ⁇
- M 2.
- processing block 570 calculates ⁇ (k) to minimize the combined error term
- 2
- ⁇ ⁇ ( k + 1 ) ⁇ ⁇ ( k ) + ⁇ 2 ⁇ ⁇ ⁇ ⁇ e ⁇ ( k ) ⁇ 2 ⁇ / ⁇ ⁇ ( 3 )
- ⁇ is a positive scaling factor smaller than 1.
- the value of ⁇ affects the convergence rate and the stability of adaptive polarization tracking, and is typically chosen to be smaller than 0.1, for example, between about 0.03 and about 0.01.
- Eq. (4) implies that, in this configuration, processing block 570 attempts to rotate signals r v and r h to equalize energy levels for signals z v and z h (because
- this configuration is effective even in the presence of large chromatic dispersion because both polarizations are equally distorted in the optical transmission link.
- FIG. 6 shows a block diagram of a processing block 680 that can be used in processing block 480 of FIG. 4 according to one embodiment of the invention. More specifically, processing block 480 would typically have two instances of processing block 680 , one for each polarization.
- Processing block 680 has a finite impulse response (FIR) filter 682 composed of (A) n ⁇ 1 serially connected D-type flip flops (delay elements) 683 , where n is a positive integer greater than 1 ; (B) n weighting blocks 685 , c j ; and (C) a summation block 687 , ⁇ .
- FIR finite impulse response
- n 39. If one redefines the values of index j in c j to run from ⁇ L to +L, then output q(k) of FIR filter 682 can be expressed as follows:
- k denotes the signaling interval and c i are the weighting coefficients used in the corresponding weighting blocks.
- the output of FIR filter 682 is applied to a squaring module 684 that determines the energy of that output by calculating
- 2 q(k)q*(k). It can be shown that, because the output of squaring module 684 represents the energy of the modulated optical carrier, it is substantially free of detrimental phase terms, such as the frequency offset, linewidth, and phase noise.
- prior-art equalization algorithms such as the least mean squares (LMS) algorithm
- LMS least mean squares
- the output of squaring module 684 is applied to a slicer 686 configured to compare each received value with one threshold value (in the case of OOK) or with two or more threshold values (in the case of PAM) to generate the output value ( ⁇ circumflex over (d) ⁇ (k)) for the corresponding signaling interval.
- the threshold value(s) used in slicer 686 can be calculated, e.g., by statistically analyzing the output of squaring module 684 over a relatively large number (e.g., >100) of signaling intervals.
- the values generated by squaring module 684 will form two or more clusters.
- Each threshold value can be set, for example, at the mid-point between the centers of mass of the two adjacent clusters or at some other value chosen to result in an acceptably small probability of slicing error.
- Processing block 680 can be configured to use a sliding-window statistical analysis, according to which a predetermined number of the most recent values are statistically analyzed to adaptively determine and/or update the slicing threshold(s) for slicer 686 .
- Weighting coefficients c j used in the weighting blocks of FIR filter 682 are set by a weight updating unit (WUU) 690 .
- WUU 690 calculates the weighting coefficients by attempting to minimize the cost function, J(k), defined as follows:
- WUU 690 implements the minimization by calculating the weighting coefficients using the following recursive formula:
- ⁇ right arrow over (c) ⁇ ( k+ 1) ⁇ right arrow over (c) ⁇ ( k ) ⁇ e ( k ) q *( k ) ⁇ right arrow over (y) ⁇ ( k ) (7)
- the value of ⁇ affects the convergence rate and the stability of the equalization algorithm, and is typically chosen to be smaller than 0.1, for example, between about 0.03 and about 0.01.
- switch SI in processing block 680 is configured to feed WUU 690 with e(k) values calculated based on the (known) training data, d(k).
- the training mode can be used, e.g., to set the initial values of weighting coefficients in FIR filter 682 .
- switch SI is configured to feed WUU 690 with e(k) values calculated based on the decoded data, d(k).
- FIGS. 7A-B graphically compare the performance of receiver 200 when DSP 260 is configured to use (i) the prior-art LMS algorithm and (ii) the above-described SEA algorithm. More specifically, the transmitter was configured to apply to link 100 a non-return-to-zero (NRZ) OOK signal having a bit rate of 10 Gb/s. Link 100 had a group velocity dispersion value of about 16 ns/nm. The linewidth of the optical carrier and of the LO signal was about 10 MHz. The frequencies of the optical carriers in the data signal and the LO signal differed by about 1 GHz.
- NRZ non-return-to-zero
- FIG. 7A shows an eye diagram corresponding to the prior-art LMS algorithm
- FIG. 7B shows an eye diagram corresponding to the SEA algorithm.
- the prior-art LMS algorithm substantially fails, whereas the SEA algorithm advantageously produces an eye diagram having a relatively widely open “eye” indicative of a relatively low BER value.
- Embodiments of the invention may be implemented as circuit-based processes, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack.
- various functions of circuit elements may also be implemented as processing blocks in a software program.
- Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer.
- each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.
- Couple refers to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements.
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Optical Communication System (AREA)
Abstract
Description
- This application claims priority from U.S. Provisional Patent Application No. 60/876,617 filed Dec. 22, 2006, which is incorporated herein by reference in its entirety.
- 1. Field of the Invention
- The present invention relates to optical communication equipment and, more specifically, to signal processing in optical receivers of on-off keying (OOK) and/or pulse amplitude modulation (PAM) signals.
- 2. Description of the Related Art
- This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
- A polarization-diverse intradyne receiver operates on an optical signal having two independently modulated polarization components. The receiver may employ sophisticated signal processing to recover the two corresponding data streams. However, there are two major challenges that hinder the advancement of this technique. First, it is very difficult to synchronize a local laser (also often referred to as a reference optical carrier or local oscillator) with the received modulated optical carrier in terms of phase and frequency because the optical frequencies are relatively large (typically, on the order of 200 THz). Second, it is very difficult to align the polarization of the local laser with that of the received optical signal because the polarization of the latter is affected by the optical transmission link and tends to change over time.
- According to one embodiment, an optical receiver adapted to recover on-off keying (OOK) or pulse amplitude modulation (PAM) data carried by a modulated optical carrier comprises an optical detector adapted to produce a sequence of vector pairs having first and second digital vectors indicative of complex values of first and second polarization components, respectively, of the modulated optical carrier at a corresponding sampling time. The optical receiver further comprises a digital processor that is connected to receive the sequence and is adapted to perform a rotation on each pair in a manner that tends to compensate for polarization rotation produced by transmitting the modulated optical carrier from an optical transmitter thereof to the optical receiver. The digital processor is further adapted to estimate values of the OOK or PAM data encoded onto each of the first and second polarization components based on the vectors produced by the rotation in a manner responsive to values of energy errors in the estimated values.
- According to one embodiment, an apparatus of the invention comprises an optical receiver adapted to recover OOK or PAM data carried by a modulated optical carrier. The optical receiver comprises an optical detector adapted to produce a sequence of first digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time. The optical receiver further comprises a digital processor being connected to receive the sequence of said first digital vectors and being adapted to estimate values of said OOK or PAM data based on the received sequence of said first digital vectors in a manner responsive to values of energy errors in said estimated values.
- According to another embodiment, an apparatus of the invention comprises an optical receiver adapted to recover data carried by a modulated optical carrier. The optical receiver comprises an optical detector adapted to produce a sequence of vector pairs having first and second digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time, and each second digital vector being indicative of a complex value of a different second polarization component of the modulated optical carrier at the sampling time. The optical receiver further comprises a digital processor being connected to receive the sequence of said vector pairs and being adapted to perform a rotation on a pair from said sequence in a manner that tends to compensate for polarization rotation produced during transmission of the modulated optical carrier from an optical transmitter thereof to the optical receiver, the digital processor being configured to recover said data from vectors resulting from the performed rotation.
- According to yet another embodiment, a method of recovering OOK or PAM data carried by a modulated optical carrier comprises the step of producing a sequence of first digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time. The method further comprises the step of estimating values of said OOK or PAM data based on the sequence of said first digital vectors in a manner responsive to values of energy errors in said estimated values.
- According to yet another embodiment, a method of recovering data carried by a modulated optical carrier comprises the step of producing a sequence of vector pairs having first and second digital vectors, each first digital vector being indicative of a complex value of a first polarization component of the modulated optical carrier at a corresponding sampling time, and each second digital vector being indicative of a complex value of a different second polarization component of the modulated optical carrier at the sampling time. The method further comprises the step of performing a rotation on a vector pair from said sequence in a manner that tends to compensate for polarization rotation produced during transmission of the modulated optical carrier from an optical transmitter thereof to an optical receiver, which is adapted to recover said data from vectors resulting from the performed rotation.
- Other aspects, features, and benefits of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:
-
FIG. 1 shows a representative optical transmission link; -
FIG. 2 shows a block diagram of an optical receiver that can be coupled to the optical transmission link ofFIG. 1 according to one embodiment of the invention; -
FIG. 3 shows a block diagram of an optical-to-electrical (O/E) converter that can be used in the optical receiver ofFIG. 2 according to one embodiment of the invention; -
FIG. 4 shows a block diagram of a digital signal processor (DSP) that can be used in the optical receiver ofFIG. 2 according to one embodiment of the invention; -
FIG. 5 shows a block diagram of a polarization-tracking block that can be used in the DSP ofFIG. 4 according to one embodiment of the invention; -
FIG. 6 shows a block diagram of a signal-equalization block that can be used in the DSP ofFIG. 4 according to one embodiment of the invention; and -
FIGS. 7A-B graphically compare the performance of the receiver shown inFIG. 2 when it is configured to use (i) a prior-art algorithm and (ii) a signal-equalization algorithm according to one embodiment of the invention. -
FIG. 1 shows a representativeoptical transmission link 100 that couples an optical transmitter (Tx) and an optical receiver (Rx). The transmitter applies to link 100 an optical signal (modulated optical carrier) 102 having two independently modulated polarization components, illustratively a vertically polarizedcomponent 102 v and a horizontally polarizedcomponent 102 h. The transmitter generates both components of signal 102 using either on-off keying (OOK) or pulse amplitude modulation (PAM) signaling. While transmitting optical signal 102 to the receiver,link 100 imposes onto it various impairments, some of which are discussed in more detail below. These impairments transform optical signal 102 into anoptical signal 152 having a vertically polarizedcomponent 152 v and a horizontally polarizedcomponent 152 h.Optical signal 152 is applied to the receiver, where appropriate signal processing is performed to recover the information encoded onto signal 102 at the transmitter. - One impairment that might be imposed onto optical signal 102 in
link 100 is polarization distortion (PD), which is represented inFIG. 1 byblock 110. More specifically,link 100 typically has an optical fiber. Different sections of the fiber typically have different principal states of polarization (PSPs) that are not necessarily aligned with the polarization components of signal 102. This misalignment causes each of the polarization components to generally have a non-zero projection on each of the PSPs, which mixes the polarization components and alters the polarization state of the signal. In addition, the PSPs tend to change over time, e.g., due to varying physical conditions inlink 100. This temporal variation causes the relationship betweensignal components 102 v-h and 152 v-h to vary over time as well. - Another impairment that might be imposed onto optical signal 102 in
link 100 is chromatic dispersion (CD), which is represented inFIG. 1 byblock 120. It is known that different spectral components of an optical pulse travel in an optical fiber at slightly different speeds because the index of refraction of the fiber core is a function of frequency (wavelength). As a result, the optical pulse broadens as it propagates along the fiber. Two parameters commonly used to characterize chromatic dispersion are the first-order group velocity dispersion (GVD), which is measured in ps/km/nm, and the second-order GVD, which is measured in ps/km/nm2. In the optical domain, both the first- and second-order GVD are linear effects. However, in the electrical domain, i.e., upon optical-to-electrical (O/E) conversion usually performed at the receiver, the GVD becomes a nonlinear effect because a typical photo-detector measures the square of the electric field, and not the electric field itself. - Chromatic dispersion is usually static and can be effectively compensated, as known in the art, by utilizing a special dispersion-compensation module. However, such a module might be relatively expensive. Furthermore, the dispersion-compensation module might add unwanted latency to the link's performance, which usually unfavorably affects the quality of service (QoS). It is also possible that the dispersion-compensation module does not fully cancel the chromatic dispersion accrued in
optical link 100, andsignal 152 still has in it some residual amount of chromatic dispersion. - Yet another impairment that might be imposed onto optical signal 102 in
link 100 is polarization-mode dispersion (PMD), which is represented inFIG. 1 byblock 130. PMD is caused by different propagation speeds of two orthogonal polarization modes due to fiber birefringence. Fiber birefringence is typically caused by deviations in the shape of the fiber core from a perfect cylinder and can be induced, e.g., by stress, bending, and/or temperature gradients. The PMD phenomenon is dynamic in nature and the PMD amount tends to change over time. - PMD is usually characterized by differential group delay (DGD), a parameter measured in ps/km0.5 and related to PMD-induced optical pulse broadening. PMD is frequency-dependent. First-order PMD is a frequency-independent component of (this frequency-dependent) PMD and is responsible for linear inter-symbol interference (ISI) in the electrical domain (e.g., at the output of the photo-detector). Second-order (as well as higher-order) PMD is a frequency-dependent component of PMD and is responsible for optical-pulse broadening similar to that produced by the corresponding order of chromatic dispersion.
- Yet another impairment that might be imposed onto optical signal 102 in
link 100 is optical noise, which is represented inFIG. 1 byadders 140 v-h.Link 100 usually employs one or more optical amplifiers. While boosting the signal intensity, an optical amplifier might also add incoherent amplified spontaneous-emission (ASE) noise (often referred to as optical noise). In the classical limit, i.e., when the signal and noise involve a relatively large number of photons, the optical noise can be modeled as complex additive white Gaussian noise (AWGN) in the optical field. The optical noise is added to each of the two orthogonally polarized components of the amplified signal.Adder 140 v indicates the addition of AWGN nv(t) to a vertically polarizedcomponent 132 v of an intermediate optical signal 132. Similarly,adder 140 h indicates the addition of AWGN nh(t) to a horizontally polarizedcomponent 132 h of signal 132. - Yet another impairment that might be imposed onto optical signal 102 in
link 100 is spectral distortion, which is represented inFIG. 1 by filteringblock 150. For example, link 100 might contain add/drop multiplexers, optical routers, and/or wavelength-division multiplexing (WDM) filters. Each of these elements is characterized by a corresponding transmission spectrum that affects the spectral content of an optical signal passing therethrough. - One skilled in the art will appreciate that the above-described impairments might accrue in
link 100 through either localized or distributed mechanisms, or through a combination of both types of mechanisms. The order, in which the impairment blocks are shown inFIG. 1 , is for illustration only. Neither is it intended to imply that different impairments are added in any particular order, nor that they accrue in separate physical sections oflink 100. -
FIG. 2 shows a block diagram of an optical receiver 200 that can be coupled to optical transmission link 100 (FIG. 1 ) according to one embodiment of the invention. Receiver 200 has an O/E converter 220 adapted to convertoptical signal 152 received fromlink 100 into electrical signals 238 a-d. Receiver 200 further has a plurality ofamplifiers 240, each receiving a corresponding one of electrical signals 238 a-d and being coupled to a corresponding analog-to-digital converter (ADC) 250. EachADC 250 samples the output of thecorresponding amplifier 240 at sampling frequency fs to produce a corresponding one of digital signals 252 a-d. In a preferred configuration, frequency fs is the same as the signaling rate insignal 152 or an integer multiple of that rate. Digital signals 252 a-d are applied to a digital signal processor (DSP) 260 that processes them to recover the two data streams originally encoded onto optical signal 102 at the transmitter (seeFIG. 1 ). The recovered data streams are then output fromDSP 260 via anoutput signal 262. - O/
E converter 220 implements polarization-sensitive intradyne detection using areference signal 212 generated by alocal oscillator 210. Polarization beam splitters (PBSs) 222 a-b decomposesignals components components optical hybrid 226. In one embodiment, O/E converter 220 is an integrated planar waveguide circuit. - In
optical hybrid 226, each ofpolarization components component 212 v and one copy ofcomponent 212 h usingphase shifters 228 a-b, respectively. The various copies are optically mixed as shown inFIG. 2 using fouroptical signal mixers 230, and the outputs of the mixers (the optical mixtures) are detected by eight photo-detectors (e.g., photodiodes) 236. Photo-detectors 236 are arranged in pairs, as shown inFIG. 2 , and the output of each photo-detector pair is a corresponding one of electrical signals 238 a-d. One skilled in the art will appreciate thatelectrical signal 238 a is a measure of the real part of vertically polarizedcomponent 152 v in the complex plane defined byLO signal 212. Similarly,electrical signal 238 b is a measure of the imaginary part of vertically polarizedcomponent 152 v in that complex plane;electrical signal 238 c is a measure of the real part of horizontally polarizedcomponent 152 h in that complex plane; andelectrical signal 238 d is a measure of the imaginary part of horizontally polarizedcomponent 152 h in that complex plane. - Due to a frequency mismatch between the optical carriers of
signals signal 152, digital signals 252 a-d are convoluted signals having contributions corresponding to both of the originaloptical signals 102 v-h (seeFIG. 1 ). The signal processing ofDSP 260 substantially de-convolutes digital signals 252 a-d to recover the two data streams originally encoded ontosignals 102 v-h. One skilled in the art will appreciate that the signal impairments described above in reference toFIG. 1 introduce signal distortions that make the data recovery relatively difficult. However, the signal processing methods implemented inDSP 260 are robust with respect to those signal distortions and advantageously provide a lower bit-error rate (BER) than comparable prior-art methods. For example, optical receiver 200 is capable of operating without employing conventional frequency and/or phase estimation forLO 210 because OOK and PAM signaling relies on photon energy (rather than phase) to carry bit information. -
FIG. 3 shows a block diagram of an O/E converter 320 that can be instead of O/E converter 220 of optical receiver 200 according to another embodiment of the invention. O/E converter 320 is generally analogous to O/E converter 220 and may be constructed using the same basic elements. However, O/E converter 320 is designed to perform optical signal mixing before polarization splitting, whereas O/E converter 220 performs polarization splitting before optical signal mixing. As a result, anoptical hybrid 326 used in O/E converter 320 employs onephase shifter 228, compared to two such phase shifters employed inoptical hybrid 226.Optical hybrid 326 also employs twooptical signal mixers 230, compared to four such mixers employed inoptical hybrid 226. On the other hand, O/E converter 320 employs four PBSs 222, compared to two such PBSs employed in O/E converter 220. - Exemplary optical hybrids that may be suitable for use in O/
E converters optical hybrids -
FIG. 4 shows a block diagram of aDSP 460 that can be used asDSP 260 according to one embodiment of the invention.DSP 460 has two serially connected processing blocks 470 and 480.Processing block 470 is a polarization tracking (PolTrack) block designed to perform, in the electrical domain, adaptive polarization correction that substantially cancels the effect of polarization distortion in optical transmission link 100 (seeblock 110 inFIG. 1 ).Processing block 480 implements a signal equalization algorithm (SEA) that enablesDSP 460 to recover the data carried bysignal 152. -
Processing block 470 operates on complex numbers. Input signal rv received by processingblock 470 carries a stream of complex numbers, each composed of a real part and an imaginary part supplied by digital signals 252 a-b, respectively. Similarly, input signal rh carries a stream of complex numbers, each composed of a real part and an imaginary part supplied bydigital signals 252 c-d, respectively. Each of output signals zv and zh generated by processingblock 470 similarly carries complex numbers. In the event that the orientation of principal polarization components ofsignal 152 ofFIG. 3 is aligned with the orientation of the polarization states of PBSs 222, the processing ofprocessing block 470 can be bypassed and input signals rv and rh can be applied directly toprocessing block 480. The orientations of the polarization states of PBSs 222 and of principal polarization components ofsignal 152 can be aligned, for example, if the polarization distortion inoptical transmission link 100 is relatively small or if an appropriately configured optical polarization rotator processes signal 152 before it is applied to receiver 200. -
FIG. 5 shows a block diagram of aprocessing block 570 that can be used asprocessing block 470 ofFIG. 4 according to one embodiment of the invention. In the absence of polarization-dependent loss (PDL) and polarization-dependent phase variation inoptical transmission link 100, the polarization distortion (seeblock 110 inFIG. 1 ) in the link consists substantially of polarization rotation and can be described using a Jones matrix.Processing block 570 has avector rotator 572 that rotates signals rv and rh (which are vectors in the complex plane) by angle θ, the value of which is chosen so as to substantially cancel the polarization rotation imposed inlink 100. Eq. (1) shows the corresponding Jones matrix used invector rotator 572 and how it is applied to signals rv and rh to obtain signals zv and zh: -
- where k is a running index that denotes signaling intervals or time slots.
-
Processing block 570 further has an error-tracking module (ETM) 574 configured to provide the θ(k) values tovector rotator 572. For each signaling interval,ETM 574 calculates signal errors (e(k)) for both vertical and horizontal polarizations using Eqs. (2a)-(2b): -
e v(k)=a v(k)−|zv(k)|2 (2a) -
e h(k)=a h(k)−|zh(k)|2 (2b) - where a(k) is defined as follows: (i) in a training mode, when the encoded data sequence dv,h(k) is known, av,h(k)=dv,h(k) and (ii) in a normal operating mode, when the encoded data sequence is not known, av,h(k)={circumflex over (d)}v,h(k), where {circumflex over (d)}v,h(k) represents decoded data obtained from signal 262 (see
FIG. 2 orFIG. 4 ). For M-level PAM signaling, dv,h(k) or {circumflex over (d)}v,h(k) ε{0,1, . . . , M−1} For OOK signaling, M=2. - In one configuration,
processing block 570 calculates θ(k) to minimize the combined error term |e(k)|2=|ev(k)|2+|eh(k)|2 using the following recursive formula: -
- where γ is a positive scaling factor smaller than 1. The value of γ affects the convergence rate and the stability of adaptive polarization tracking, and is typically chosen to be smaller than 0.1, for example, between about 0.03 and about 0.01. Using Eq. (2) and the fact that ∂{eh}/∂θ=−∂{ev}/∂θ, Eq. (3) can be expressed as follows:
-
θ(k+1)=θ(k)+γ[|z v|2 −|z H|2 ]∂{e h}/∂θ (4) - Eq. (4) implies that, in this configuration, processing block 570 attempts to rotate signals rv and rh to equalize energy levels for signals zv and zh (because |z|2 is a measure of energy). In the case of conventional OOK encoding, with the same bits on both polarizations, this configuration is effective even in the presence of large chromatic dispersion because both polarizations are equally distorted in the optical transmission link. However, caution needs to be exercised while using this configuration with polarization-multiplexed OOK.
-
FIG. 6 shows a block diagram of aprocessing block 680 that can be used inprocessing block 480 ofFIG. 4 according to one embodiment of the invention. More specifically,processing block 480 would typically have two instances ofprocessing block 680, one for each polarization. InFIG. 6 , the signal applied to processing block 680 is designated with a generic subscript x (=v or h) to make the drawing descriptive of both polarizations. Ifprocessing block 680 is used inprocessing block 480 coupled to polarization-tracking block 470, then yx=zx (seeFIG. 4 ). However, if polarization-tracking block 470 is bypassed or not present, then yx=rx. In the description that follows, the polarization subscript is omitted altogether for the sake of simplicity. -
Processing block 680 has a finite impulse response (FIR)filter 682 composed of (A) n−1 serially connected D-type flip flops (delay elements) 683, where n is a positive integer greater than 1; (B) n weighting blocks 685, cj; and (C) asummation block 687, Σ. In a representative embodiment, n=39. If one redefines the values of index j in cj to run from −L to +L, then output q(k) ofFIR filter 682 can be expressed as follows: -
- where k denotes the signaling interval and ci are the weighting coefficients used in the corresponding weighting blocks. The output of
FIR filter 682 is applied to asquaring module 684 that determines the energy of that output by calculating |q(k)|2=q(k)q*(k). It can be shown that, because the output of squaringmodule 684 represents the energy of the modulated optical carrier, it is substantially free of detrimental phase terms, such as the frequency offset, linewidth, and phase noise. In contrast, prior-art equalization algorithms, such as the least mean squares (LMS) algorithm, do not have a module analogous to squaringmodule 684 and rely on the values representing the amplitude of the modulated optical carrier, and not its energy, to determine the data values and to generate a feedback error signal for setting the weighting coefficients. More details on the prior-art LMS algorithm can be found, e.g., in J. H. Winters, “Equalization in Coherent Lightwave Systems Using Microwave Waveguides,” J. Lightwave Technology, vol. 7, No. 5, pp. 813-815, May 1989, the teachings of which are incorporated herein by reference. - The output of squaring
module 684 is applied to aslicer 686 configured to compare each received value with one threshold value (in the case of OOK) or with two or more threshold values (in the case of PAM) to generate the output value ({circumflex over (d)}(k)) for the corresponding signaling interval. The threshold value(s) used inslicer 686 can be calculated, e.g., by statistically analyzing the output of squaringmodule 684 over a relatively large number (e.g., >100) of signaling intervals. Typically, the values generated by squaringmodule 684 will form two or more clusters. Each threshold value can be set, for example, at the mid-point between the centers of mass of the two adjacent clusters or at some other value chosen to result in an acceptably small probability of slicing error.Processing block 680 can be configured to use a sliding-window statistical analysis, according to which a predetermined number of the most recent values are statistically analyzed to adaptively determine and/or update the slicing threshold(s) forslicer 686. - Weighting coefficients cj used in the weighting blocks of
FIR filter 682 are set by a weight updating unit (WUU) 690.WUU 690 calculates the weighting coefficients by attempting to minimize the cost function, J(k), defined as follows: -
J(k)=[e(k)]2 (6) - where e(k) is given by Eq. (2). In one configuration,
WUU 690 implements the minimization by calculating the weighting coefficients using the following recursive formula: -
{right arrow over (c)}(k+1)={right arrow over (c)}(k)−βe(k)q*(k){right arrow over (y)}(k) (7) - where β is a positive scaling factor smaller than 1; {right arrow over (c)}(k)=[c−L(k), . . . , ci(k), . . . , cL(k)]T, where superscript T means transposed; and {right arrow over (y)}(k)=[y(k+L), . . . , y(k), . . . , y(k−L)]T. The value of β affects the convergence rate and the stability of the equalization algorithm, and is typically chosen to be smaller than 0.1, for example, between about 0.03 and about 0.01.
- In a training mode, switch SI in
processing block 680 is configured to feedWUU 690 with e(k) values calculated based on the (known) training data, d(k). The training mode can be used, e.g., to set the initial values of weighting coefficients inFIR filter 682. In a regular operating mode, switch SI is configured to feedWUU 690 with e(k) values calculated based on the decoded data, d(k). -
FIGS. 7A-B graphically compare the performance of receiver 200 whenDSP 260 is configured to use (i) the prior-art LMS algorithm and (ii) the above-described SEA algorithm. More specifically, the transmitter was configured to apply to link 100 a non-return-to-zero (NRZ) OOK signal having a bit rate of 10 Gb/s.Link 100 had a group velocity dispersion value of about 16 ns/nm. The linewidth of the optical carrier and of the LO signal was about 10 MHz. The frequencies of the optical carriers in the data signal and the LO signal differed by about 1 GHz. Receiver 200 was configured to have 39 taps (i.e, n=39) and to run at a sampling speed of about 20 gigasamples per second.FIG. 7A shows an eye diagram corresponding to the prior-art LMS algorithm, andFIG. 7B shows an eye diagram corresponding to the SEA algorithm. As can be seen, the prior-art LMS algorithm substantially fails, whereas the SEA algorithm advantageously produces an eye diagram having a relatively widely open “eye” indicative of a relatively low BER value. - While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although embodiments of the invention have been described in reference to OOK or PAM signals, certain aspects of the invention, e.g., digital polarization tracking can similarly be used with other modulation formats. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.
- Embodiments of the invention may be implemented as circuit-based processes, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing blocks in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer.
- Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.
- It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.
- Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.
- Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”
- Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements.
Claims (20)
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/959,960 US8078065B2 (en) | 2006-12-22 | 2007-12-19 | Polarization tracking and signal equalization for optical receivers configured for on-off keying or pulse amplitude modulation signaling |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US87661706P | 2006-12-22 | 2006-12-22 | |
US11/959,960 US8078065B2 (en) | 2006-12-22 | 2007-12-19 | Polarization tracking and signal equalization for optical receivers configured for on-off keying or pulse amplitude modulation signaling |
Publications (2)
Publication Number | Publication Date |
---|---|
US20080152363A1 true US20080152363A1 (en) | 2008-06-26 |
US8078065B2 US8078065B2 (en) | 2011-12-13 |
Family
ID=39542970
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US11/959,960 Expired - Fee Related US8078065B2 (en) | 2006-12-22 | 2007-12-19 | Polarization tracking and signal equalization for optical receivers configured for on-off keying or pulse amplitude modulation signaling |
Country Status (1)
Country | Link |
---|---|
US (1) | US8078065B2 (en) |
Cited By (12)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20100028011A1 (en) * | 2008-07-30 | 2010-02-04 | Noriaki Kaneda | Digital clock and data recovery scheme |
US20100054761A1 (en) * | 2008-08-28 | 2010-03-04 | Young-Kai Chen | Monolithic coherent optical detectors |
US20100158521A1 (en) * | 2008-12-18 | 2010-06-24 | Alcatel-Lucent Usa Inc. | Optical mixer for coherent detection of polarization-multiplexed signals |
US20100245837A1 (en) * | 2009-03-30 | 2010-09-30 | Fujitsu Limited | Polarization interferometer, optical module, and optical receiver |
US20100329677A1 (en) * | 2009-06-29 | 2010-12-30 | Noriaki Kaneda | Symbol Timing Recovery in Polarization Division Multiplexed Coherent Optical Transmission System |
CN102308500A (en) * | 2009-02-23 | 2012-01-04 | 爱斯福公司 | All-optical, phase sensitive optical signal sampling |
US20130058649A1 (en) * | 2011-05-31 | 2013-03-07 | Huawei Technologies Co., Ltd. | Method and apparatus for processing optical signals |
US8588565B2 (en) | 2009-03-20 | 2013-11-19 | Alcatel Lucent | Coherent optical detector having a multifunctional waveguide grating |
US20170155448A1 (en) * | 2015-11-30 | 2017-06-01 | Futurewei Technologies, Inc. | Frequency Domain Optical Channel Estimation |
US20170201330A1 (en) * | 2016-01-08 | 2017-07-13 | Google Inc. | In-band optical interference mitigation for direct-detection optical communication systems |
US10623217B1 (en) | 2019-05-29 | 2020-04-14 | Nvidia Corp. | Proportional AC-coupled edge-boosting transmit equalization for multi-level pulse-amplitude modulated signaling |
US11650340B2 (en) | 2020-12-01 | 2023-05-16 | Nokia Solutions And Networks Oy | Detection of seismic disturbances using optical fibers |
Families Citing this family (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US8340619B1 (en) * | 2009-02-13 | 2012-12-25 | Marvell International Ltd. | Phase synchronization of phased array or multiple receiver/transmitter systems |
KR20110005575A (en) * | 2009-07-10 | 2011-01-18 | 한국전자통신연구원 | Digital equalization apparatus and method for coherent optical receiver |
EP2375603B1 (en) * | 2010-02-05 | 2018-05-23 | Xieon Networks S.à r.l. | Clock recovery method and clock recovery arrangement for coherent polarisation multiplex receivers |
US9236973B2 (en) * | 2012-04-12 | 2016-01-12 | Futurewei Technologies, Inc. | Linear dispersion polarization-time codes and equalization in polarization multiplexed coherent optical system |
US9281915B2 (en) | 2013-01-17 | 2016-03-08 | Alcatel Lucent | Optical polarization demultiplexing for a coherent-detection scheme |
US9154231B2 (en) | 2013-01-17 | 2015-10-06 | Alcatel Lucent | Generation of an optical local-oscillator signal for a coherent-detection scheme |
JP6759742B2 (en) * | 2016-06-16 | 2020-09-23 | 富士通株式会社 | Receiver and setting method |
Citations (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20020008506A1 (en) * | 1998-11-11 | 2002-01-24 | Juichi Nakada | Electric power measuring method, system using the same and computer-readable medium |
US20020042802A1 (en) * | 2000-06-07 | 2002-04-11 | Yukihiko Mogi | FIR filter and setting method of coefficient thereof |
US20020186435A1 (en) * | 2000-09-26 | 2002-12-12 | Isaac Shpantzer | System and method for orthogonal frequency division multiplexed optical communication |
US6714742B1 (en) * | 1999-05-20 | 2004-03-30 | University Of Southern California | Polarization-division multiplexing based on power encoding of different polarization channels |
US20040146099A1 (en) * | 2003-01-28 | 2004-07-29 | Phyworks Limited | Receiver |
US20040264598A1 (en) * | 2003-06-25 | 2004-12-30 | Interdigital Technology Corporation | Method and system for adjusting the amplitude and phase characteristics of real and imaginary signal components of complex signals processed by an analog radio transmitter |
US20050196176A1 (en) * | 2004-03-08 | 2005-09-08 | Han Sun | Equalization strategy for dual-polarization optical transport system |
US20070092259A1 (en) * | 2005-10-21 | 2007-04-26 | Nortel Networks Limited | Polarization compensation in a coherent optical receiver |
US20080152362A1 (en) * | 2006-12-22 | 2008-06-26 | Ut-Va Koc | Adaptive polarization tracking and equalization in coherent optical receivers |
US20090034981A1 (en) * | 2007-08-03 | 2009-02-05 | Sumitomo Electric Industries, Ltd. | Optical transceiver wtih equalizing function and a method to setup the optical transceiver |
-
2007
- 2007-12-19 US US11/959,960 patent/US8078065B2/en not_active Expired - Fee Related
Patent Citations (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20020008506A1 (en) * | 1998-11-11 | 2002-01-24 | Juichi Nakada | Electric power measuring method, system using the same and computer-readable medium |
US6714742B1 (en) * | 1999-05-20 | 2004-03-30 | University Of Southern California | Polarization-division multiplexing based on power encoding of different polarization channels |
US20020042802A1 (en) * | 2000-06-07 | 2002-04-11 | Yukihiko Mogi | FIR filter and setting method of coefficient thereof |
US20020186435A1 (en) * | 2000-09-26 | 2002-12-12 | Isaac Shpantzer | System and method for orthogonal frequency division multiplexed optical communication |
US20040146099A1 (en) * | 2003-01-28 | 2004-07-29 | Phyworks Limited | Receiver |
US20040264598A1 (en) * | 2003-06-25 | 2004-12-30 | Interdigital Technology Corporation | Method and system for adjusting the amplitude and phase characteristics of real and imaginary signal components of complex signals processed by an analog radio transmitter |
US20050196176A1 (en) * | 2004-03-08 | 2005-09-08 | Han Sun | Equalization strategy for dual-polarization optical transport system |
US20070092259A1 (en) * | 2005-10-21 | 2007-04-26 | Nortel Networks Limited | Polarization compensation in a coherent optical receiver |
US20080152362A1 (en) * | 2006-12-22 | 2008-06-26 | Ut-Va Koc | Adaptive polarization tracking and equalization in coherent optical receivers |
US20090034981A1 (en) * | 2007-08-03 | 2009-02-05 | Sumitomo Electric Industries, Ltd. | Optical transceiver wtih equalizing function and a method to setup the optical transceiver |
Cited By (23)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20100028011A1 (en) * | 2008-07-30 | 2010-02-04 | Noriaki Kaneda | Digital clock and data recovery scheme |
US8095019B2 (en) * | 2008-07-30 | 2012-01-10 | Alcatel Lucent | Digital clock and data recovery scheme |
US20100054761A1 (en) * | 2008-08-28 | 2010-03-04 | Young-Kai Chen | Monolithic coherent optical detectors |
US20100158521A1 (en) * | 2008-12-18 | 2010-06-24 | Alcatel-Lucent Usa Inc. | Optical mixer for coherent detection of polarization-multiplexed signals |
WO2010080307A1 (en) | 2008-12-18 | 2010-07-15 | Alcatel-Lucent Usa Inc. | Optical mixer for coherent detection of polarization-multiplexed signals |
CN102308500A (en) * | 2009-02-23 | 2012-01-04 | 爱斯福公司 | All-optical, phase sensitive optical signal sampling |
US20120134667A1 (en) * | 2009-02-23 | 2012-05-31 | Exfo Inc. | All-Optical, Phase Sensitive Optical Signal Sampling |
US8768180B2 (en) * | 2009-02-23 | 2014-07-01 | Exfo, Inc. | All-optical, phase sensitive optical signal sampling |
US8588565B2 (en) | 2009-03-20 | 2013-11-19 | Alcatel Lucent | Coherent optical detector having a multifunctional waveguide grating |
US20100245837A1 (en) * | 2009-03-30 | 2010-09-30 | Fujitsu Limited | Polarization interferometer, optical module, and optical receiver |
US20100329677A1 (en) * | 2009-06-29 | 2010-12-30 | Noriaki Kaneda | Symbol Timing Recovery in Polarization Division Multiplexed Coherent Optical Transmission System |
US8655191B2 (en) | 2009-06-29 | 2014-02-18 | Alcatel Lucent | Symbol timing recovery in polarization division multiplexed coherent optical transmission system |
EP2685642A2 (en) * | 2011-05-31 | 2014-01-15 | Huawei Technologies Co., Ltd. | Method and device for processing optical signals |
US20130058649A1 (en) * | 2011-05-31 | 2013-03-07 | Huawei Technologies Co., Ltd. | Method and apparatus for processing optical signals |
EP2685642A4 (en) * | 2011-05-31 | 2014-09-03 | Huawei Tech Co Ltd | Method and device for processing optical signals |
US8995831B2 (en) * | 2011-05-31 | 2015-03-31 | Huawei Technologies Co., Ltd. | Method and apparatus for processing optical signals |
US20170155448A1 (en) * | 2015-11-30 | 2017-06-01 | Futurewei Technologies, Inc. | Frequency Domain Optical Channel Estimation |
US9680574B1 (en) * | 2015-11-30 | 2017-06-13 | Futurewei Technologies, Inc. | Frequency domain optical channel estimation |
US20170201330A1 (en) * | 2016-01-08 | 2017-07-13 | Google Inc. | In-band optical interference mitigation for direct-detection optical communication systems |
US9998235B2 (en) * | 2016-01-08 | 2018-06-12 | Google Llc | In-band optical interference mitigation for direct-detection optical communication systems |
US10084547B2 (en) | 2016-01-08 | 2018-09-25 | Google Llc | In-band optical interference mitigation for direct-detection optical communication systems |
US10623217B1 (en) | 2019-05-29 | 2020-04-14 | Nvidia Corp. | Proportional AC-coupled edge-boosting transmit equalization for multi-level pulse-amplitude modulated signaling |
US11650340B2 (en) | 2020-12-01 | 2023-05-16 | Nokia Solutions And Networks Oy | Detection of seismic disturbances using optical fibers |
Also Published As
Publication number | Publication date |
---|---|
US8078065B2 (en) | 2011-12-13 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US8078065B2 (en) | Polarization tracking and signal equalization for optical receivers configured for on-off keying or pulse amplitude modulation signaling | |
Chen et al. | Dual polarization full-field signal waveform reconstruction using intensity only measurements for coherent communications | |
Kikuchi | Fundamentals of coherent optical fiber communications | |
US8886051B2 (en) | Skew compensation and tracking in communications systems | |
US9020364B2 (en) | Optical receiver having a signal-equalization capability | |
EP1570593B1 (en) | Coherent optical detection and signal processing method and system | |
Tsukamoto et al. | Unrepeated transmission of 20-Gb/s optical quadrature phase-shift-keying signal over 200-km standard single-mode fiber based on digital processing of homodyne-detected signal for group-velocity dispersion compensation | |
US8433205B2 (en) | Crosstalk-free high-dimensional constellations for dual-polarized nonlinear fiber-optic communications | |
US8306440B2 (en) | Polarization diversity receiver systems and methods with polarization mode dispersion mitigation | |
US20060034618A1 (en) | Adaptive optical equalization for chromatic and/or polarization mode dispersion compensation | |
US11799560B2 (en) | Asymmetric direct detection of optical signals | |
Do et al. | Data-aided OSNR estimation for QPSK and 16-QAM coherent optical system | |
Chen et al. | Full-field, carrier-less, polarization-diversity, direct detection receiver based on phase retrieval | |
Fan et al. | Transceiver IQ imperfection monitor by digital signal processing in coherent receiver | |
KR101931957B1 (en) | Optical transmission method and system using polarization-time coding for polarization diversity multiplexed optical transmission | |
Mori et al. | 200-km transmission of 100-Gbit/s 32-QAM dual-polarization signals using a digital coherent receiver | |
Faruk et al. | Multi-impairments monitoring from the equalizer in a digital coherent optical receiver | |
Xie et al. | Increasing polarization-mode dispersion tolerance of coherent receivers by joint optimization of chromatic dispersion and butterfly equalizers | |
Van den Borne et al. | Electrical PMD compensation in 43-Gb/s POLMUX-NRZ-DQPSK enabled by coherent detection and equalization | |
Fattah et al. | Electronic signal processing for cancelation of optical systems impairments | |
Noé et al. | Realtime digital signal processing in coherent optical PDM-QPSK and PDM-16-QAM transmission | |
Geyer et al. | Optimization of the chromatic dispersion equalizer of a 43Gb/s realtime coherent receiver | |
WO2023073927A1 (en) | Digital signal processing circuit, method, receiver, and communication system | |
US20220393771A1 (en) | Mitigation of equalization-enhanced phase noise in a coherent optical receiver | |
Westhäuser et al. | Optical equalization of pmd-induced penalties in 112 gbit/s metro networks |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
AS | Assignment |
Owner name: LUCENT TECHNOLOGIES INC., NEW JERSEY Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:KOC, UT-VA;REEL/FRAME:020354/0512 Effective date: 20080104 |
|
FEPP | Fee payment procedure |
Free format text: PAYOR NUMBER ASSIGNED (ORIGINAL EVENT CODE: ASPN); ENTITY STATUS OF PATENT OWNER: LARGE ENTITY |
|
ZAAA | Notice of allowance and fees due |
Free format text: ORIGINAL CODE: NOA |
|
ZAAB | Notice of allowance mailed |
Free format text: ORIGINAL CODE: MN/=. |
|
AS | Assignment |
Owner name: ALCATEL-LUCENT USA INC., NEW JERSEY Free format text: MERGER;ASSIGNOR:LUCENT TECHNOLOGIES INC.;REEL/FRAME:027047/0930 Effective date: 20081101 |
|
AS | Assignment |
Owner name: ALCATEL LUCENT, FRANCE Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:ALCATEL-LUCENT USA INC.;REEL/FRAME:027069/0868 Effective date: 20111013 |
|
STCF | Information on status: patent grant |
Free format text: PATENTED CASE |
|
FPAY | Fee payment |
Year of fee payment: 4 |
|
MAFP | Maintenance fee payment |
Free format text: PAYMENT OF MAINTENANCE FEE, 8TH YEAR, LARGE ENTITY (ORIGINAL EVENT CODE: M1552); ENTITY STATUS OF PATENT OWNER: LARGE ENTITY Year of fee payment: 8 |
|
FEPP | Fee payment procedure |
Free format text: MAINTENANCE FEE REMINDER MAILED (ORIGINAL EVENT CODE: REM.); ENTITY STATUS OF PATENT OWNER: LARGE ENTITY |
|
LAPS | Lapse for failure to pay maintenance fees |
Free format text: PATENT EXPIRED FOR FAILURE TO PAY MAINTENANCE FEES (ORIGINAL EVENT CODE: EXP.); ENTITY STATUS OF PATENT OWNER: LARGE ENTITY |
|
STCH | Information on status: patent discontinuation |
Free format text: PATENT EXPIRED DUE TO NONPAYMENT OF MAINTENANCE FEES UNDER 37 CFR 1.362 |
|
FP | Lapsed due to failure to pay maintenance fee |
Effective date: 20231213 |